Well, I think the answer is "Yes, but". A couple of issues that come to my mind:

1) Does Bartolini make any pickups that are drop-in replacements for those on an L-2000? If so (they list G&L 5-string replacement pickups in their online info at www.bartolini.net but I don't see anything for 4-strings) then you can drop them in the existing holes, otherwise you may need to route to get them in.

2) The switches and things on the G&L are passive (except for the passive/active switch for the preamp). You may want to keep the series/parallel switch (if your new pickups are split-coil or dual coil) but will probably replace everything else with the Bart preamp. If there are more control holes from the G&L controls than you need for the Bartolini then just plug any extras.

3) Is the Bart preamp on a circuit board? If so you'll want to check whether the G&L control cavity is deep enough for it.

4) Are the controls for the Bart rigidly attached to a circuit board? If so then the holes probably won't line up with the G&L holes. If not then as long as the wires/cables are long enough you can use the existing holes.

5) Is the Bart preamp 18V? My Climax had a separate body cavity (under the control cover) for the battery so you might have to find another location for a second battery.

That's all the possible issues I can think of. You might want to do a thorough shielding job on the L-2000 while it's all torn apart. Also, if you end up having to make new holes a control plate or pickguard that extended to that location could cover the extra ones.

Depending on how many issues you have to deal with to swap the electronics you might want to consider selling the G&L and getting a different bass that has Barts. Or maybe doing a Warmoth bass with Barts. Or seeing if you can get a replacement body routed for the pickups/electronics you want (maybe from Warmoth, although I don't know if they can/will to a G&L neck pocket).

If you go through with it I'd be willing to buy the old G&L pickups and electronics from you provided they were working. I love the sound of G&Ls (I won't try to talk you out of it if that's what you want to do - but I'd never modify a G&L with non-G&L pickups, maybe the preamp but not the pickups).

Heck, I'd buy the body/electronics from you in whole if you wanted to put a replacement body with Barts on it. I'd love to try an L-2000 with a Moses neck and that would get me most of the way there.

If you think that Barts plus the Aguilar preamp (sorry I kept calling it a Bart preamp, poor reading comprehension on my part) is necessary to give you the sound you want then get that setup. The list of things I put together is coming from someone who thinks about these types "Frankenstein" things but has never actually done one. They are all just things to verify and take care of when performing the change. I'm sure a good luthier/tech could handle all of them with no problem (not that it would be cheap, but a good guy could handle them all no problem).

I know, given my personal set of current bass experience, that a G&L L-2000 (or ASAT) is necessary for me to get the sounds I like. I'm selling a Fender P and getting an L-2000 so that I have a bass that makes the sounds I like. I'm even going to, if I have to, put a Moses neck on it so I can have the bass I want with a neck with dimensions/profile I can be comfortable with. If the Barts/Aguilar setup is that same "it" for you then get that setup.

My only real suggestion is weigh your options and decide what to do. If you have to hack up an L-2000 to put the Barts/Aguilar in it you may have your "ultimate bass". However you will have demolished a lot of the resale value of the instrument. That may not be a problem for you but there's risk. Since you haven't tried the Bart/Aguilar combo in an L-2000 (who has?) you may not like it. And then you are stuck with a highly customized bass that may not appeal to anyone else. I think a big part of the G&L's appeal is its character, a lot of which is provided by the electronics. If you start with something that can take the Barts/Aguilar as a drop-in replacement then you can try them, switch things and eventually put everything back stock if you decide to go with something else.

If you really loved how the Pentabuzz sounded, then maybe what you should really do is figure out a way to get it (or a similar one). Save up, sell your L2K, etc. If that's what you really want you won't be happy with something else.

Or alternatively you could try other basses similar to the Pentabuzz that you feel are "close" and only need a little "tweak" (Barts & Aguilar) to get close. I don't know exactly what would be comparable - Peavey Cirrus? Ibanez BTB?

Can't help you with the strings. I've never put anything other than nickel rounds (non-taper) of various gauges and brands on my previous L-2000s. I always thought there was plenty of "growl" but then again my idea of "growl" may be different than yours and I've never tried a Pentabuzz. On the L2000 my idea of clarity is parallel, active, treble & bass full up. I actually think that my MTD Beast (single coil Bartolini jazz pickups) has greater clarity (as do the SB-1, LB-100 and Precisions I've tried). I've come to associate "clarity" with a single coil picking up the string - whether it be a jazz-like pickup or a precision-like pickup there's a single coil picking up the motion of each string.

Sorry for writing a book but anybody who wants can suck me into a topic - just put L-2000 in there somewhere and I'll be unable to resist.

Oh yeah, one more thing (like you wanted to hear another thing from me). Did you try the Pentabuzz through your amp? The reason I ask is, like you mention, the amp can make a big difference. My setup sounds different through my 1x12 combo, through my (now gone) head & 1x15 cab and through the PA speakers (4 x 15). To fairly compare basses you've got to have the same rig. Everything (including my L2000s) can sound crappy through my headphone amp and pretty much everything sounded good through the head&cab (although they sounded different). And for the final trial-by-fire you've got to try them in your playing situation.